The present invention relates to filters, and more particularly, to permanent magnet filters of the type used to remove corrosion products from flow lines in power plants and other industrial processes.
Although corrosion is of concern in most industrial processes, such concern is magnified in fluid processes involving highly toxic or radioactive materials, such as nuclear power plants. Corrosion products enter a nuclear steam generator through the feed train and are also developed within the steam generator itself. The steam exiting the steam generator contains little or no corrosion products. The nuclear steam generator therefore collects a large portion of the corrosion products produced in the system.
These corrosion products circulate within the fluid of the steam generator until they are eventually deposited on a metal surface inside the generator. The largest portion of the corrosion products eventually deposit on the tube sheet which is the lowest location within the generator. Corrosion products in the form of sludge, have accumulated to a thickness of up to 12 inches in some operating steam generators. This sludge layer reduces the heat transfer area of the steam generator and thus adversely effects the efficiency of the unit. Also, the sludge corrosively attacks the tubes and results in leakage of the radioactive primary fluid from the reactor to the steam generator.
Continuous blow-down procedures are employed in most steam generators in an attempt to reduce the concentration of corrosion products circulating within the steam generator. Also, systems are being developed whereby a portion of the fluid within the steam generator is removed, cooled and circulated through an external magnetic filter. The fluid is then returned to the system. One such technique is described in co-pending U.S. patent application Ser. No. 020,324, filed Feb. 27, 1987. Although the techniques described in that patent application represent a significant improvement over prior magnet filter techniques, further improvement would be desirable, in terms of cost-effectively increasing the intensity, and thus the filtering power of the magnetic fields.